The present invention relates generally to microelectronic dice and packaging such devices, and more particularly to a wirebond structure and method to make an electrically connection to a microelectronic die.
Microelectronic dice, semiconductor chips and the like are typically housed in a protective covering referred to as a package or packaging. Pins formed on the package are electrically connected by bond wires to corresponding pads formed in a substrate or inter-layer dielectric of the die or chip. The pins of the package are then used to connect the microelectronic die to a socket mounted on a circuit board, or the pins of the package may, in some designs, be directly connected to a circuit board or other substrate.
The pads formed in the inter-layer dielectric of the die are typically formed of copper and the bond wires are usually formed from aluminum or gold. Both copper and aluminum, when exposed to an oxidizing environment, such as air, will form a thin layer of oxide on the exposed surfaces. Copper is prone to continuos oxidation and aluminum is not. An oxide layer between the aluminum and copper connection will increase the electrical resistance of the conductive path formed by the copper pad and the aluminum bond wire. The strength of the electrical contact formed by connecting the aluminum bond wire directly to the copper pad can also be weak mechanically and deteriorate over time.
Additionally, aluminum and copper can react with one another when exposed to high temperatures such as those used in testing microelectronic dice, circuit boards and the like or in specialized applications where high temperatures and oxidizing environments may be encountered. Exposure to high temperatures, such as up to about 200xc2x0 Celsius or higher, can cause the connection formed between the aluminum bond wire and the copper pad to be unstable resulting in breakage and a disconnection of the bond or resulting in bonds that will be more susceptible to deterioration over time. High temperatures can also cause increased oxidation of the metals and an increase in the electrical resistance between the copper and aluminum bond or connection. While such high temperatures are not expected to be encountered in most applications, microelectronic dice and circuits are tested at such temperatures to insure the reliability and stability of these devices and their connections over time and under all normal environmental conditions.
One known wirebond structure 100 is shown in FIG. 1. A copper bond pad 102 is formed in an inter-layer dielectric 104 of a package 105. A silicon nitride passivation layer 106 may be formed on a surface 108 of the inter-layer dielectric 104 and over a top surface 110 of the copper bond pad 102. A polyimide passivation layer 112 may be formed over the silicon nitride layer 106. An opening 114 is formed in the polyimide layer 112 and the silicon nitride layer 106 to expose at least a portion of the top surface 110 of the copper bond pad 102. A first thin barrier layer 116 of tantalum is formed on the top surface 110 of the copper pad 102 and a second thicker layer 118 of aluminum is formed on the tantalum barrier layer 116. The tantalum barrier layer 116 and the second layer 118 of aluminum must be etched to form the wirebond structure 100. The aluminum bond wire 120 is then attached to the second layer 118. Accordingly, the wirebond structure 100 requires a multiple layer interface including the first barrier layer 116 to contact the copper pad 102 and the second layer 118 of aluminum to make contact with the aluminum bond wire 120. Forming and etching each of these layers 116 and 118 requires additional process steps and materials that increase the cost and time of production.
Accordingly, for the reasons stated above and for other reasons that will become apparent upon reading and understanding the present specification, there is a need for a wirebond structure that includes only a single layer of conductive material between the aluminum bond wire and the copper pad. Additionally, there is a need for a wirebond structure that forms a stable, robust bond or connection between the aluminum bond wire and the copper pad and that can withstand oxidizing environments and high temperatures and that maintains its integrity over time and under most operating conditions.